The role of hydrothermal processes and the formation of the J-M reef and associated rocks of olivine-bearing zone I of the Stillwater Complex, Montana

Abstract

Several lines of evidence, including hydrous melt inclusions and unusually Cl-rich apatite, have been used to suggest that the reappearance of olivine and PGE-sulfide of the J-M Reef in the Stillwater Complex, Montana, is due to fluid infiltration and hydration melting. This study builds upon the hydration melting model using the programs MELTS and PELE with Stillwater bulk rock compositions for the original protolith. Cl-bearing phases are not modeled by MELTS and thus simple oxide mixtures of either a pure H₂O or a H₂O + Na₂O “faux brine” are added to norite, gabbronorite, and melanorite protoliths at 1050 °C at 2 kbar pressure, conditions for which the nominally “dry” protolith is > 95% solid. Incongruent hydration melting results in up to 37% olivine produced in the melanorite. The olivine Fo content is a function of the partial melt retained on cooling, and ranges between 76 and 86, overlapping the natural range of olivine compositions observed in the rocks. Modeling with the PELE program, which includes a silicate liquid Cl component, sulfur species, and a more complex C-O–H-S fluid, suggests that, for CO₂-rich fluids, fluid metal concentrations on the order of 25 ppm Pt, 75 ppm Pd, 0.03 wt.% Cu, and 0.20 wt.% Ni at a fluid/rock mass ratio of ~ 0.25 are needed to account for the observed ore grades. Sulfide and ore metals are readily remobilized for more H₂O-rich fluids, consistent with heterogeneous distribution of sulfide and regionally variable ore grades.